Water treatment system

Abstract

According to one embodiment, a water treatment system includes: a membrane separation device and a water softening device including a valve unit switching between a softening process in which soft water is obtained by passing raw water through a cation exchange resin bed downward and a regeneration process in which a whole of the cation exchange resin bed is regenerated by generating an opposite flow of a regenerant by supplying the regenerant from both sides of a top portion and a bottom portion of the cation exchange resin bed and collecting a liquid at an intermediate portion of the cation exchange resin bed, and a regenerant supply supplying the regenerant in a volume which gives a regeneration level of 1 to 6 eq/L-R, to a hardness leakage prevention region set in a predetermined depth on the bottom portion of the cation exchange resin bed, in the regeneration process.

What is claimed is: 
     
       1. A water treatment system, comprising:
 a water softening device configured to produce soft water having a hardness of equal to or lower than 0.8 mg CaCO 3 /L from raw water having a hardness of equal to or lower than 500 mg CaCO 3 /L; and 
 a membrane separation device configured to produce permeate water from the soft water, wherein: 
 the water softening device comprises a pressure tank including a top portion screen, an intermediate portion screen, and a bottom portion screen, the pressure tank having:
 a supporting bed provided in association with the bottom portion screen, including filtration gravel so as to filter soft water and a regenerant; and 
 a cation exchange resin bed to which the raw water or the regenerant is supplied, the cation exchange resin bed having a depth D 1 , and the cation exchange resin bed being provided on the supporting bed in association with the intermediate portion screen, 
 
 a valve unit configured to switch between a softening process in which the soft water is obtained by passing the raw water through the cation exchange resin bed downward while supplying the raw water from the top portion screen, and the obtained water is filtered through the supporting bed and collected from the bottom portion screen, and a regeneration process in which the cation exchange resin bed is regenerated by generating an opposite flow of the regenerant by supplying a first portion of the regenerant from the top screen portion and a second portion of the regenerant from the bottom screen portion while filtering with the supporting bed and collecting a liquid at the intermediate portion, and 
 a regenerant supply unit configured to supply the regenerant in a volume which gives a regeneration level of 1 to 6 eq/L-R, with respect to the cation exchange resin bed, in the regeneration process; and 
 the membrane separation device comprises:
 a reverse osmosis membrane module configured to separate the soft water into the permeate water and concentrated water; 
 a pressure pump configured to be driven at a rotation speed corresponding to an input drive frequency and to intake the soft water to discharge the soft water toward the reverse osmosis membrane module; 
 an inverter equipment configured to output the drive frequency corresponding to an input current value signal to the pressure pump; and 
 a control unit configured to calculate the drive frequency of the pressure pump by using a physical quantity in the water treatment system such that a flow rate of the permeate water becomes a preset target flow rate value and to output a current value signal corresponding to a calculated value of the drive frequency to the inverter equipment; 
 
 a bottom portion of the cation exchange resin bed is regenerated in the regeneration process, the bottom portion having a depth D 2  corresponding to D 1 / 15 <D 2 <D 1 / 3  with respect to the depth D 1  of the cation exchange resin bed; and 
 the reverse osmosis membrane module includes a reverse osmosis membrane on a surface of which a skin layer of a negative electric characteristic made of a bridged wholly aromatic polyamide is formed, the reverse osmosis membrane having a water permeability coefficient of 1.5×10 −11  m 3 ·m −2 ·s −1 ·Pa −1  or higher when a sodium chloride aqueous solution of concentration 500 mg/L. pH 7.0, and a temperature 25° C. is supplied at an operating pressure 0.7 MPa and a recovery rate 15%. 
 
     
     
       2. The water treatment system according to  claim 1 , wherein:
 the membrane separation device further comprises a flow rate detecting unit configured to detect a flow rate of the permeate water, and 
 the control unit is configured to: 
 (i) calculate the drive frequency of the pressure pump by a speed type digital PID algorithm such that a detected flow rate value of the flow rate detecting unit becomes the preset target flow rate value; and 
 (ii) output a calculated value signal corresponding: to a calculated value of the drive frequency to the inverter equipment. 
 
     
     
       3. The water treatment system according to  claim 1 , wherein:
 the membrane separation device further comprises:
 a pressure detecting unit configured to detect a discharge pressure of the pressure pump, and 
 a temperature detecting unit configured to detect a temperature of the soft water, the permeate water, or the concentrated water, and 
 
 the control unit is configured to:
 (i) calculate a discharge pressure of the pressure pump based on a detected temperature value of the temperature detecting unit, a water permeability coefficient at a reference temperature of the reverse osmosis membrane module, and the preset target flow rate value; 
 (ii) set a calculated value of the discharge pressure as a target pressure value; 
 (iii) calculate the drive frequency of the pressure pump by a speed type digital PID algorithm such that a detected pressure value of the pressure detecting unit becomes the target pressure value; and 
 (iv) output a calculated value signal corresponding to a calculated value of the drive frequency to the inverter equipment. 
 
 
     
     
       4. The water treatment system according to  claim 1 , wherein:
 the membrane separation device further comprises a temperature detecting unit configured to detect a temperature of the soft water, the permeate water, or the concentrated water, and 
 the control unit is configured to:
 (i) calculate a discharge pressure of the pressure pump based on a detected temperature value of the temperature detecting unit, a water permeability coefficient value at a reference temperature of the reverse osmosis membrane module, and the preset target flow rate value; 
 (ii) calculate the drive frequency of the pressure pump based on a calculated value of the discharge pressure; and 
 (iii) output a calculated value signal corresponding to a calculated value of the drive frequency to the inverter equipment. 
 
 
     
     
       5. The water treatment system according to  claim 1 , wherein:
 the membrane separation device further comprises:
 a temperature detecting unit configured to detect a temperature of the soft water, the permeate water, or the concentrated water, and 
 a drain valve configured to adjust a drain flow rate of the concentrated water that is drained to an outside of the membrane separation device, and the control unit is configured to: 
 (i) calculate a permissible concentration rate of silica in the concentrated water based on silica concentration of the raw water or the soft water obtained in advance and silica solubility determined by a detected temperature value of the temperature detecting unit; 
 (ii) calculate the drain flow rate based on a calculated value of the permissible concentration rate and the preset target flow rate value of the permeate water; and 
 (iii) control the drain valve such that an actual drain flow rate of the concentrated water becomes a calculated value of the drain flow rate. 
 
 
     
     
       6. The water treatment system according to  claim 2 , wherein:
 the membrane separation device further comprises:
 a temperature detecting unit configured to detect a temperature of the soft water, the permeate water, or the concentrated water, and 
 a drain valve configured to adjust a drain flow rate of the concentrated water that is drained to an outside of the membrane separation device, and 
 
 the control unit is configured to:
 (i) calculate a permissible concentration rate of silica in the concentrated water based on silica concentration of the raw water or the soft water obtained in advance and silica solubility determined by a detected temperature value of the temperature detecting unit; 
 (ii) calculate the drain flow rate based on a calculated value of the permissible concentration rate and the preset target flow rate value of the permeate water; and 
 (iii) control the drain valve such that an actual drain flow rate of the concentrated water becomes a calculated value of the drain flow rate. 
 
 
     
     
       7. The water treatment system according to  claim 3 , wherein:
 the membrane separation device further comprises:
 a temperature detecting unit configured to detect a temperature of the soft water, the permeate water, or the concentrated water, and 
 a drain valve configured to adjust a drain flow rate of the concentrated water that is drained to an outside of the membrane separation device, and 
 
 the control unit is configured to:
 (i) calculate a permissible concentration rate of silica in the concentrated water based on silica concentration of the raw water or the soft water obtained in advance and silica solubility determined by a detected temperature value of the temperature detecting unit; 
 (ii) calculate the drain flow rate based on a calculated value of the permissible concentration rate and the preset target flow rate value of the permeate water; and 
 (iii) control the drain valve such that an actual drain flow rate of the concentrated water becomes a calculated value of the drain flow rate. 
 
 
     
     
       8. The water treatment system according to  claim 4 , wherein:
 the membrane separation device further comprises:
 a temperature detecting unit configured to detect a temperature of the soft water, the permeate water, or the concentrated water, and 
 a drain valve configured to adjust a drain flow rate of the concentrated water that is drained to an outside of the membrane separation device, and 
 
 the control unit is configured to:
 (i) calculate a permissible concentration rate of silica in the concentrated water based on silica concentration of the raw water or the soft water obtained in advance and silica solubility determined by a detected temperature value of the temperature detecting unit; 
 (ii) calculate the drain flow rate based on a calculated value of the permissible concentration rate and the preset target flow rate value of the permeate water; and 
 (iii) control the drain valve such that an actual drain flow rate of the concentrated water becomes a calculated value of the drain flow rate. 
 
 
     
     
       9. The water treatment system according to  claim 1 , wherein:
 the membrane separation device further comprises:
 a hardness measuring unit configured to measure calcium hardness of the soft water, and 
 a drain valve configured to adjust a drain flow rate of the concentrated water that is drained to an outside of the membrane separation device, and 
 
 the control unit is configured to:
 (i) calculate a permissible concentration rate of calcium carbonate in the concentrated water based on calcium carbonate solubility obtained in advance and a measured hardness value of the hardness measuring unit; 
 (ii) calculate a drain flow rate based on a calculated value of the permissible concentration rate and the preset target flow rate value of the permeate water; and 
 (iii) control the drain valve such that an actual drain flow rate of the concentrated water becomes a calculated value of the drain flow rate. 
 
 
     
     
       10. The water treatment system according to  claim 2 , wherein:
 the membrane separation device further comprises:
 a hardness measuring unit configured to measure calcium hardness of the soft water, and 
 a drain valve configured to adjust a drain flow rate of the concentrated water that is drained to an outside of the membrane separation device, and 
 
 the control unit is configured to:
 (i) calculate a permissible concentration rate of calcium carbonate in the concentrated water based on calcium carbonate solubility obtained in advance and a measured hardness value of the hardness measuring unit; 
 (ii) calculate a drain flow rate based on a calculated value of the permissible concentration rate and the preset target flow rate value of the permeate water; and 
 (iii) control the drain valve such that an actual drain flow rate of the concentrated water becomes a calculated value of the drain flow rate. 
 
 
     
     
       11. The water treatment system according to  claim 3 , wherein:
 the membrane separation device further comprises:
 a hardness measuring unit configured to measure calcium hardness of the soft water, and 
 a drain valve configured to adjust a drain flow rate of the concentrated water that is drained to an outside of the membrane separation device, and 
 
 the control unit is configured to:
 (i) calculate a permissible concentration rate of calcium carbonate in the concentrated water based on calcium carbonate solubility obtained in advance and a measured hardness value of the hardness measuring unit; 
 (ii) calculate a drain flow rate based on a calculated value of the permissible concentration rate and the preset target flow rate value of the permeate water; and 
 (iii) control the drain valve such that an actual drain flow rate of the concentrated water becomes a calculated value of the drain flow rate. 
 
 
     
     
       12. The water treatment system according to  claim 4 , wherein:
 the membrane separation device further comprises:
 a hardness measuring unit configured to measure calcium hardness of the soft water, and 
 a drain valve configured to adjust a drain flow rate of the concentrated water that is drained to an outside of the membrane separation device, and 
 
 the control unit is configured to:
 (i) calculate a permissible concentration rate of calcium carbonate in the concentrated water based on calcium carbonate solubility obtained in advance and a measured hardness value of the hardness measuring unit; 
 (ii) calculate a drain flow rate based on a calculated value of the permissible concentration rate and the preset target flow rate value of the permeate water; and 
 (iii) control the drain valve such that an actual drain flow rate of the concentrated water becomes a calculated value of the drain flow rate. 
 
 
     
     
       13. The water treatment system according to  claim 1 , wherein:
 the membrane separation device further comprises:
 an electric conductivity measuring unit configured to measure electric conductivity of the permeate water, and 
 a drain valve configured to adjust a drain flow rate of the concentrated water that is drained to an outside of the membrane separation device, and 
 
 the control unit is configured to control the drain flow rate from the drain valve such that a measured electric conductivity value of the electric conductivity measuring unit becomes a preset target electric conductivity value. 
 
     
     
       14. The water treatment system according to  claim 2 , wherein:
 the membrane separation device further comprises:
 an electric conductivity measuring unit configured to measure electric conductivity of the permeate water, and 
 a drain valve configured to adjust a drain flow rate of the concentrated water that is drained to an outside of the membrane separation device, and 
 
 the control unit is configured to control the drain flow rate from the drain valve such that a measured electric conductivity value of the electric conductivity measuring unit becomes a preset target electric conductivity value. 
 
     
     
       15. The water treatment system according to  claim 3 , wherein:
 the membrane separation device further comprises:
 an electric conductivity measuring unit configured to measure electric conductivity of the permeate water, and 
 a drain valve configured to adjust a drain flow rate of the concentrated water that is drained to an outside of the membrane separation device, and 
 
 the control unit is configured to control the drain flow rate from the drain valve such that a measured electric conductivity value of the electric conductivity measuring unit becomes a preset target electric conductivity value. 
 
     
     
       16. The water treatment system according to  claim 4 , wherein:
 the membrane separation device further comprises:
 an electric conductivity measuring unit configured to measure electric conductivity of the permeate water, and 
 a drain valve configured to adjust a drain flow rate of the concentrated water that is drained to an outside of the membrane separation device, and 
 
 the control unit is configured to control the drain flow rate from the drain valve such that a measured electric conductivity value of the electric conductivity measuring unit becomes a preset target electric conductivity value. 
 
     
     
       17. The water treatment system according to  claim 1 , wherein:
 the depth D 1  of the cation exchange resin bed is within the range of 300 mm to 1500 mm, and 
 the depth D 2  of the bottom potion is 100 mm.